RSpec is an extremely convenient tool for structuring and writing specs, and I use it on all my ruby-projects.
One set of built-in helpers I’d like to cheer on a bit are
let!, as they can greatly simplify setting up state for multiple examples. They exist to provide a simple and convenient way to create and share state between examples. When not using these methods, specs with a bit of shared state typically look like this:
This is easy to get started with, and when there are only a few variables in the
before block, things are fine. But if you have a typo in an example where you intended to use the
@user instance variable in an example, but you accidentally typed
@usr, you just get a
nil value, instead of a helpful error message regarding an unknown indentifier. This minor problem can be avoided by using
let instead, as you get an unknown indentifier error instead of a
nil value. Another issue is that all variables in the
before block are evaluated, whether or not they are actually used in the examples.
The excellent official RSpec 2 documentation has the following to say on
let, before showing clear examples of use:
letto define a memoized helper method. The value will be cached across multiple calls in the same example but not across examples.
letis lazy-evaluated: it is not evaluated until the first time the method it defines is invoked. You can use
let!to force the method’s invocation before each example.
The fact that results are both memoized and lazy-evaluated means no extra effort is expended if the value is not used in a given example. Performance win!
Please use the correct helper method - I’ve seen examples like this:
The exact same thing can be expressed directly and become more readable, just by adding a
let! helpers are available in from versions
rspec 1.3.1, and
rspec-core 2.0.0 onwards. In a nutshell, this means they are available for use in Rails 2 and Rails 3 projects. Take them for a spin if you haven’t already, and tell me how it goes.
I recommend having a quick look at the Rspec 2.5 documentation now, as you’ll probably discover one or more neat new solutions to recurring issues when writing specs.
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